1. Field of the Invention
The present invention relates to a radio communication control station, a radio communication terminal, a home agent, and a radio communication method for implementing IP communication in a composite radio communication network comprising a plurality of radio communication networks.
2. Prior Art
On account of the development of radio communication networks including mobile phone networks and the universalization of the Internet, the demand of Internet communication by means of radio communication terminals is rising. Such a demand has been satisfied, e.g., by i-mode (Trademark of NTT DoCoMo Inc. Tokyo, Japan) and the rapid popularization of i-mode is remarkable in this country.
In accordance with the i-mode technique, however, terminals are not given Internet protocol (IP) addresses, which are terminal identifiers for the Internet. Accordingly, a node on the Internet-side cannot carry out self-initiated transmission to the mobile phone terminals over IP although the mobile phone terminals can access to the Internet via the gateway interconnecting the mobile phone network and the Internet, using the hyper text transfer protocol (HTTP) or the like.
If a computer terminal plugged with a mobile telephone handset by a modem accesses to the Internet via the mobile telephone network, the terminal is temporarily allocated an IP address by an Internet access provider. In this case, a node on the Internet-side can carry out self-initiated transmission to the terminal over IP if the node knows the temporary IP address somehow. However, even in this case, the terminal is not permanently allocated an IP address.
The Internet has been designed and constructed with geographically fixed sub-networks and terminals, and packet routing has been operated on the IP address which represents the fixed sub-network where the terminal belongs. Therefore, even if a terminal is given an IP address, it cannot communicate with another party via another sub-network when the IP address is different from and is not void in the address system of the other party's sub-network.
The Mobile Internet Protocol (mobile IP) is known as a technique for solving the above-described issue on IP address. In accordance with mobile IP, an inherent or permanent IP address is allocated to a mobile terminal as well as fixed terminals. When the mobile terminal is linked with a sub-network different from its home sub-network, where the mobile terminal originally subscribes, the mobile terminal is assigned another IP address for the newly visited sub-network. Accordingly, the mobile terminal not only has the original or inherent IP address, but also the temporary IP address that is effective in the visited sub-network. Since the relationship between the inherent IP address and the new IP address is registered in the home sub-network, IP packets destined for the inherent IP address are transferred to the assigned IP address that is effective in the latest stage, whereby the destination terminal can receive the IP packets successfully.
The home sub-network where a mobile terminal subscribes is referred to as a home network for the mobile terminal. The home network includes a mechanism called home agent (HA) that monitors the relationship between the inherent IP address and the newest IP address for every mobile terminal subscribing to the home network itself, and transfers IP packets using the newest IP addresses.
JP-A-2000-253069 discloses a technique for ensuring quality of service although mobile IP is utilized. In accordance with mobile IP, IP packets are encapsulated so as to be transferred to the sub-network visited by the destination terminal. At this encapsulation, the inherent IP packets are modified to be adapted for the sub-network, which the destination mobile terminal currently visits. The technique of JP-A-2000-253069 keeps the attributes of the quality of service indicated in the inherent IP packets effective at the modification. The use of mobile IP has a merit that IP packets may be successfully transferred on the basis of the newest IP addresses corresponding to inherent IP addresses. However, problems has been indicated. For example, the cost and latency are increased for transferring packets from the HA to the destination terminal.
On the other hand, demands for high rate transmission using radio communication terminals and flexible mobility support for mobile terminals have been increased. Therefore, the studies on stratosphere platform, MMAC (multiple model adaptive control), and so on have been progressed.
Generally, when a high frequency band is used for implementing high rate communication, the electromagnetic attenuation and other problems occur due to the severe directivity and much diffusion and absorption at reflections, resulting great difficulty in mobility support although the data propagation rate can be enhanced. Accordingly, it is difficult to satisfy requirements of all radio communication terminals by a sole network. Thus, a radio overlay network (composite radio communication network) including a plurality of radio networks has been studied. In the radio overlay network, one or more constituent networks are selectively used in accordance with requirements of a user or application.
However, in conventional composite radio communication networks, it is impossible for radio communication terminals such as mobile phone handsets to implement IP communication.